The present invention relates to a circuit configuration for a brake system including an anti-locking control, wherein a balanced brake pressure is attainable in the rear wheel brakes through control according to the select-low principle, that is, in response to the rear wheel which is the first to show a locking tendency.
As is well known, an anti-locking control system (ALC) is intended to preclude, in panic decelerations or on slippery roads, a locking of the wheels in order to maintain driving stability and steerability of the automotive vehicle. In the event of locking rear wheels, the driving stability is subject to swerving, whereas in the event of locking front wheels, the steerability of the automotive vehicle is lost. However, in order to attain a short stopping distance, the brake pressure through the anti-locking system is reduced just to an extent preventing the wheel from locking and enabling it to absorb sufficient lateral forces. As the lateral forces which a wheel is able to absorb rapidly decrease with an increasing brake slip and as a excessively low lateral stability of the rear wheels involves the danger of swerving, a control of the brake pressure in the rear wheel brakes according to the select-low principle has proved to be satisfactory and is commonly used in modern brake systems. According to this principle, the rear wheels are always decelerated with the same pressure determined by the "low" wheel, that is, by the rear wheel exhibiting the poorer road contact, and, therefore, being the first wheel tending to lock. The "high" rear wheel, hence, normally receives a slightly lower brake pressure than it could handle in view of the locking tendency. This wheel is, therefore, capable of transferring a relatively high lateral force.
A disadvantage involved with the select-low principle is that the high wheel, the brake pressure of which is not determined by its own rotating pattern but rather by the low wheel, does not contribute enough to deceleration if--for example due to a poor bleeding condition, a low pad friction coefficient etc.--the brake moment, in response to the brake pressure, is substantially lower than the one on the second rear wheel. With an equal friction coefficient in righ/left and straight-forward driving, a brake moment difference (which may be substantial) always will occur which in unfavorable circumstances, results in a stopping distance larger than required. Unfavorable conditions of this type are especially encounted in snow and ice conditions and at a relatively high load on the rear axle as compared with that on the front axle, that is, in situations in which deceleration is required over a relatively extended period of time.
It is the object of the present invention to overcome the disadvantages described and to provide a circuit configuration ensuring the advantages involved with a select-low control on the rear wheel with no need for accepting a high brake moment difference in unbalanced conditions of the rear wheel brakes in the described situations.